There is no /etc/vpnc/vpnc-script. Checking the ipk file I found the following.

root@GL-MT1300:~# opkg files vpnc
Package vpnc (0.5.3.r550-9) is installed on root and has the following files:
/usr/sbin/vpnc
/usr/sbin/vpnc-disconnect
/lib/netifd/proto/vpnc.sh
/lib/upgrade/keep.d/vpnc
/etc/vpnc/default.conf

root@GL-MT1300:~# opkg files vpnc-scripts
Package vpnc-scripts (20151220-2) is installed on root and has the following files:
/lib/netifd/vpnc-script

So you can do this to make a symbol link. Then try again.

ln -s /lib/netifd/vpnc-script /etc/vpnc/